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CFX vs. Fluent
I'm expanding into using CFD sim software & am looking at CFX & Fluent. Can people who know both tell me the strengths & weaknesses of each?
I notice CFX got bought a couple years back by Ansys, which I have used (but I prefer IDEAS). Anyone who used/uses CFX before & after, any good or bad changes to the product or support because of that? |
Re: CFX vs. Fluent
I know that CFX has more inbuilt models when it comes to Multiphase flows. You should wait and see what others have to say.
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Re: CFX vs. Fluent
I don't have a direct experience on CFX. However all my collegues who uses CFX tells it's more robust and accurate than FLUENT, but it's a little more difficult to be used.
Maybe it's the price to pay for accurate solutions :) Regards, ap :) |
Re: CFX vs. Fluent
i have used both the softwares, at least for combustion i won't go for fluent, in Fluent we have only two realistic models EDC and flamelet, out of which EDC is real pain in arse to get converged, and i feel as if we almost do not have it.
i am sure those who has tried EDC know what i am talking about. |
Re: CFX vs. Fluent
I would add some consideration about FLUENT.
1) FLUENT is easy to use and has a good interface both for pre-processing (GAMBIT) and for setting the case. Postprocessing is not that good, but it's ok if you consider that it's integrated in the solver graphical interface. 2) FLUENT is lacking from a mathematical point of view. It doesn't allow to choose among all discretization schemes available, even when you enable the specific option via text commands. CDS seem to be considered a tabu (they are hidden for all models except LES) at FLUENT, while everyone knows that the upwind schemes are very diffusive and highly inaccurate in many cases. Morover, if you simulate multiphase flows, you only have the phase coupled SIMPLE alghorithm for pressure velocity coupling. Nothing else is available. Always in the Eulerian model, you can only use the first order implicit discretization scheme for time integration. The second order (Crank-Nicholson) is not available. 3) FLUENT can be personalized using UDFs. UDFs are perfect if you want to do simple changes to the standard models, but they are a pain if you need to implement some complex additional model. This is mainly due to the *lack of documentation*. The UDF manual is completely *insufficient*, and if you need something you have to contact FLUENT support. In the UDF manuals you don't find many important informations, many macros are not documented, some examples are wrong (the re-implementation of the Syamlal drag model). A good UDF manual would provide just a list of the macros required to communicate with the solver and of the DEFINE functions, like in a doxygen documentation. Now, I listed some of what I consider the main lacking parts of FLUENT. However, if you want a general purpose CFD code and you're not interested in research applications, FLUENT is probably ok, even if some specific area of modeling may not be covered in the proper way. Just check if it's suitable for your purpose. Regards, ap |
Re: CFX vs. Fluent
i completely agree with you that when it comes to documentation fluent is certainly not good thing. but CFX documents are well written.
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Re: CFX vs. Fluent
Good to hear. Quality documentation is essential for a CFD code.
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Re: CFX vs. Fluent
Dear George: If you want to model free surface flows especially using VOF model; the FLOW-3D is the best.\ regards
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Re: CFX vs. Fluent
I have worked with both CFX and fluent and I find that CFX is more robust and much better for bigger simulations. The overall control on different parameters is much better in CFX and its more user friendly compared to the Fluent. The documentation is just too good. Especially for Tascflow. The best part os the post-processing, which is probably the best.
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Re: CFX vs. Fluent
So what combustion model does CFX offer? I have been working in this field for auite some time and mainly using in-house code and FLUENT.
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Re: CFX vs. Fluent
I offer that n selecting a CFD software focus on the code (engine) not post and pre processor. in solving turbulence fluid flow I prefer FLUENT.
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Re: CFX vs. Fluent
for that u can check cfx web site there you can find in detail .... but i strongly feel that it depends upon person to person that what he likes .... some people may feel with fluent at ease ..some may find cfx more apealing ... for me in fluent flamelet is one good model for combustion ..and edc though is good .. but has always troubled me .... and after reading lot of postings here about problems faced by people with edc on fluent ..i have finally made opinion that edc with fluent just sucks.
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Re: CFX vs. Fluent
I've seen and used both so I'll add a few possibly useful points.
* Fluent - Preprocessing (Gambit and Fluent GUI together) seem like they have a fast, yet somewhat antiquated (based on tcl/tk or something like that), user interface. So, if you don't care about a modern look and feel GUI then it will be fine. - Their solvers have many, many, many different models. However, their feature matrix is extremely sparse. i.e. model x works with model y but not with model z, etc... - Many of their boundary condition and source term type models are quite "engineer" oriented. i.e. enter some sort of commonly known engineering data and Fluent automatically translates that into boundary conditions or a source term usable by their solver. - Flow solver robustness is questionable on complex models (combustion, multiphase, etc..) . You will have to play around alot if that is what you are doing. - Their post processor is nice in that it is directly integrated with their solvers, but is really limited in functionality. You pretty much have to bail and use Ensight or Fieldview to do anything sophisticated. - and has many presupplied models that are basically built in "macros" fo where you can easily input a few "model" parameters that are * CFX - Preprocessing is maybe a bit slower than Fluent in terms of overall speed but it keeps on improving. CFX has a much more modern GUI based on QT, is fully integrated into ANSYS Workbench now which allows direct connections with CAD, is configurable like Fluent's solver GUI. The workflow in Workbench is really really nice and Fluent has nothing that even comes close to this. - On the other hand, Meshing with CFX is a bit of a dogs breakfast. There is CFX Mesh and ICEM Tetra/Prism and ICEM Hexa. You may have to learn all of these. They are great meshers but it sucks to have to possibly use all of them. - The solver is very robust. You will play around much less getting complex models working. That is not to say that you wont play around at all! - There are less built in, user friendly, engineering specific models in CFX (eg: Heat Exchanger Model, Fan Models, Spray Atomiser Models are a few I can think of). - In a pure tick box feature war CFX would loose, but really, who cares. - On the other hand, the feature matrix in CFX is far far far fuller than Fluent's. - However, that being said, CFX is completely flexible enough that you could type in a couple of CEL expressions and possibly 1D interpolation tables, etc... to give yourself similar functionality on your own. - Post processing wins hands down in CFX. I'd say other than parallelisation, the CFX post processor is on par with something like Fieldview or Ensight. |
Re: CFX vs. Fluent
Hi have seen and used both codes over many years.
I think commercial code utility is covered by: - models available - solver accuracy and robustness - adaptability to specific problems In terms of models: CFX-5 has models for mainline work and better models for turbomachine applications. Fluent has more models and validation in a broad range of applications including combustion, radiation, in-cylinder flows, fuel cells, acoustics, heat exchangers, etc. In terms of Solver accuracy and robustness: Both codes are about the same in accuracy...but hard to compare on an unstructured mesh as CFX is cell-vertex Fluent is cell centered. Robustness for steady problems is better in CFX but it does not usually take too much tweaking to get Fluent to converge. Fluent has more options and control for unsteady problems. In terms of adaptability: CFX has an easy to use CFX command language to do simple things. Otherwise CFX is very closed and user Fortran is not well documented and renders it unusable. Fluent UDF's are documented, archived and very flexible. Basically CFX is a more robust solver but Fluent is a more robust solution. There are my thoughts Bak_Flow |
Re: CFX vs. Fluent
Hi,
CFX has a subset of the modern combustion models. There are significant gaps like a good pdf generation method, flamelet is adiabatic, no EDC...sure it is hard to converge ALL complex physics models are. Fluent simply has more combustion models and they have been working for longer. Regards, Bak_Flow |
Re: CFX vs. Fluent
can you explain to me what do you mean by "CFX is cell-vertex", fluent i know is definitely cell centered
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Re: CFX vs. Fluent
In many cases Fluent doesn't converge or gives not accurate results due to it's poor math implementation.
I don't know how CFX is in this field. Regards, ap |
Re: CFX vs. Fluent
In my opinion UDFs are not well documented at all. Many useful macros are not cited at all, and many are just used in some example without explanation.
If you must implement complex models you often need to contact FLUENT support. Regards, ap |
Re: CFX vs. Fluent
not only udf, there are lot of things not documented, for example , there is a big problem that in sliding mesh problems after defingin the sliding interface mesh check fails (most of the time, at least with me it happened),
i asked fluent support many times, this time they asked me to execute a command on tui before defining interface to avoid this problem, i did what they said and yes it works fine, this is no where documented and i bet there are lot of people who are facing similar problems, but fluent shamelessly not willing to put this things to documents, and this is the reason when Jonas asked about splittign this forum i said to put tip and tricks section so that we can gather all such tips for fluent users who otherwise are on mercy of fluent support. |
Re: CFX vs. Fluent
Hi ap from POLITO. Yes you're completely rigth. Fluent UDF documentation is really poor. I had to look at all header files to learn many more macros and did a lot of trial and error. I'm a little disappointed about this side of Fluent. It's a powerful tool but users cannot use all Fluent power because of this. I don't ever try to talk about scheme programming with Fluent...it's a shame!I had to learn scheme commands looking examples from fluent's site...I asked some help from Fluent support about this...but...it's better not to go on...Luca
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Re: CFX vs. Fluent
Hi all,
just for fun....go over to the CFX forum and ask people how they like the documentation for the user Fortran in CFX-5. Fluent has an entire manual and many examples. It is hard to document stuff these things....believe me you are better off with Fluent in this instance...believe me. ;-) Bak_Flow |
Re: CFX vs. Fluent
Hi,
yes there are cases that Fluent is not so robust but that can be said of any other solver with the same technology. However one would have to do a 1-1 comparison to be fair...which I have done. People usually use more complex models in Fluent for combustion and radiation for example. CFX does not have some of these models. If you run 1 step mixed is burned reaction with P1 radiation in both codes...CFX will do better. If you want the right answer say it takes EDC and DO radiation....CFX does not have the models???? Most people are doing more than flow in a duct these days...fluent simply has more models! That being said I hear Fluent is going to introduce some new solver technology including a pressure based coupled solver and some faster density based solvers for high Mach flows...that should prove interesting! Regards, Bak_Flow |
Re: CFX vs. Fluent
Hey Neale,
sounds like a bit of an advert! So a bit of the stuff in CFX is bad....but keeps on improving?? Feature matrix is far far far fuller?? Ok just to level the playing field..... Would you call a code that could not do profile boundary conditions till its 7th release, still can't do transient lagrangian particles, adaption in one fluid zone, periodic elements with DP, DT, and many others a full feature matrix?? Why is Fluent so successful....it is not about the height and breadth of the feature matrix or a nice look and feel....it is about getting the features people really need working! Regards, Bak_Flow |
Re: CFX vs. Fluent
More models doesn't mean anything, if those models don't work or their solution is not accurate.
Any user with a good CFD background, is able to implement a model, the problems come when you have to solve it an accurate way. A robust solver in the hands of expert people is a good instrument, a good set of model with a poor solver is not. Regards, ap |
Re: CFX vs. Fluent
well i used fortran with cfx and i never found any problem, and in the documents its well explained how to use, it infact gives more freedom control than udfs (i can't say much though, since udf is one thing i could not make much out of it, thanks to fluent's docs), the irony is i do lot of programming with c/c++ where as my knowledge of fortran is limited.
another part of hard to doc these things: in my opinion the arguement that some thing is bad with another software does not mean that this software should not be improved. so even if we assume that cfx docs are bad and cfx is worse compared to fluent. it doesn't mean that fluent should not improve things. fluent is such a big company if they wish they can improve the quality of their docs, otherwise how it happens that your fluent support knows lot of things that you can not learn from documentation. who tells them and where do they learn from. they must learn from some documents or training. so if such things exist in docs or training, why they are not documented for normal users like you and me. i guess the license fee is pretty hefty for fluent, can't we expect even this. and when i say cfx docs are better i mean in this regard that they have tried to put as much as possible into docs. for example fluent tutorial has around 15 examples max, compared to cfx4.4 it has more than 80 examples (even more may be i am roughly saying) , and cfx4.4 is almost outdated compared to cfx5 series. the question is can you summerise you vast featured fluent solver in just 15 tutorials. if there are features we as users like to learn. and yes, we (at least me) are not so intellegent as fluent might wish we shall be. |
Re: CFX vs. Fluent
Hi I was talking about CFX-5.
CFX4 is a dead code now. I agree the user Fortran there was useful. CFX-5 is a whole other story. Regards, Bak_Flow |
Re: CFX vs. Fluent
yes, cfx5 is entirely new thing and i can not say anythign about it,
but i shall honestly admit, i never faced any convergence problem with fluent or cfx (except for EDC which gives cold flow after convergence), and i strongly believe convergence is mostly in the hand of user, if he makes grid properly and apply model properly things can go easily. so if one understands cfd he can work with naything, there are people who get even better results with their own codes. And software comparision is very debatable issue, in the end it boils down to person likes (sometimes we just like things) |
Re: CFX vs. Fluent
Why FLUENT doesn't give a full documentation?
The answer is easy. FLUENT offers consultancy services too ;-) |
Re: CFX vs. Fluent
Yes ap, and when you try to ask them something about scheme or UDF they say that service is not in your support package.Luca
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Re: CFX vs. Fluent
I know that answer very well Luca,
"UDF are not included in your support package. But you can always buy a package of support hours or attend a course" This is what was answered to me. Not a good answer, considering FLUENT cost. |
Re: CFX vs. Fluent
I'm new to Fluent, could zxaar please explain what: "execute a command on tui" means?
(from posting: zxaar , Thu, 28 Apr 2005, 3:13 a.m) Thank you. |
Re: CFX vs. Fluent
Well, I guess the best answer to solve all these problems in time, is to wait for a FREE GPL implementation of CFD code. I frankly do not see that happening anywhere in the near future, although I do believe that this will definitely happen.
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Re: CFX vs. Fluent
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sliding mesh
see if you facing negative volumes after creating interface you can try this:
before making interfaces execute this command on TUI (text user interface or fluents command prompt) (rpsetvar 'nonconformal/cell-faces 0) after this if you define the interface the mesh check usually do not fail, at least in my case has not failed so far. |
Re: CFX vs. Fluent
Did not know about it. Neat :)
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Re: CFX vs. Fluent
Hi,
in vertex based schemes the flow variables are stored at the vertices of the mesh elements. In cell centered schemes the flow variables are stored at the centers of the mesh elements. In the first, control volumes, for which the conservation equations are satisfied are built up from pieces (quadrants for quad meshes in 2D, for example) around the vertex. In the end these control volumes form a mesh as well which is called a dual mesh which has the same fundamental purpose as in the cell centered scheme. For quad/hex meshes there is very little difference between them except a bit at the boundaries. For tri/tet meshes there is a huge difference in the resulting number or degrees of freedom. Usually a factor between 3 and 4 less for the vertex based method. This makes codes hard to compare with the same mesh. Good discussion in Blazek's book...see book section of CFD-Online! Regards, Bak_Flow |
Re: CFX vs. Fluent
I have used both codes, so I was just trying to give an objective opinion rather than show a bias one way or another. There is no question that both are great products and there is no question that Fluent and CFX both have strengths and weaknesses.
I guess I did not clearly explain what I mean by feature matrix as you have misunderstood. What you have mentioned about the various features that exist in Fluent and not in CFX is simply a feature "tick box". i.e. Why does CFX not have feature A, feature B, feature C, feature D, etc... (you give examples of transient particle tracking, periodic BCs with delta P, mass flow, etc..). For sure CFX also has useful features, which Fluent does not have, that both work well and that people really need, otherwise no one would use CFX at all. However, that being said, I don't want to get in a tick box war so I immediately concede... CFX clearly looses the "tick box" battle. Rather than a "tick box", what I meant is "feature interaction". eg: feature A is great but does not work with feature B, feature C, feature D, etc... An example in Fluent of this is the periodic condition you mentioned where you specifiy the mass flow rate. This feature does not work with combustion and only works in the segregated solver. There are many other examples like this in Fluent. If you pick any one feature that both Fluent and CFX have, you would be able to run it with more models activated in CFX than you would with Fluent. It's just the way it is. I think CFX had profile BCs since 5.3, which was the second release. How one looks at this may depend on your definition of a profile BC though. Neale |
Re: CFX vs. Fluent
There is still quite a difference in hex meshes, since in a vertex scheme, integration points are located on each sub-face (the faces of each element sector around a node), whereas a cell-centered method only has integration points at the cell faces. For a hex mesh, this means CFX has 24 integration points per control volume, as opposed to 6 integration points with Fluent.
-Guy |
Re: CFX vs. Fluent
Hi Neale,
yes I agree with your point. A massive level of interations between models is nice. I am however an Engineer....somone brings me a problem to solve if there is one thing that is missing in the code for what I need....I go to the next code or figure out how to add it myself. All I am saying is that my experience has been that there is a lot missing in CFX-5...except if you want to do turbo work! So taking your example, periodic element sections with combustion.... 1. Does anybody ever need this...i am not sure...I have seen flames through tube-banks but I think the properties are varying so strongly with temp and species variation that the flow may not be periodic?? 2. What would be the Engineering solution in CFX 5? Not to put too fine of a point on it...but in versions prior to 5.7 there was cloud of points interpolaton which one could use if: 1. you did not care to get the correct profile 2. you only had a few points because it was tedious to cut-up your variables into separate files and arduous to load come-on...........it was a pathetic! Looked like a "tick box" feature to me! ;-) I don't know that feature would be the correct term for detailed boundary conditions. Hey there is an idea for marketing people around the world...start listing all the boundary conditions options, fluids, etc. Regards, Bak_Flow |
Re: CFX vs. Fluent
Yep,
but I was not really addressing the Fluent vs CFX implementations just the guy's question about centered and vertex schemes in general. ;-) On that note there is nothing preventing a Cell centered scheme from using more integration points than 1 on the faces. Formally, 1 integration point/face and 4 are both 2nd order accurate on regular grids. In practice I believe more is better but I have not read any good studies on this. Bak_Flow |
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